Later this year I will be 70. As I write that big number, a large part of my brain is refusing to accept that it is true. But my birth certificate is proof I was born on December 27, 1946.

My parents’ generation grew up thinking of their lives as a three-act drama - education, followed by employment and then retirement. On reaching 70 they were on the downward slope, joints gradually seizing up and horizons shrinking.

Later this year, Janet Street-Porter will be 70. As she writes that big number, a large part of her brain is refusing to accept that it is true. But her birth certificate is proof she was born on December 27, 1946

She spent her final days sitting in the same chair, alone in a small flat in Llandudno, eating meagre microwaved meals and watching telly, barely speaking to her neighbours. Quality of life: rubbish.

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I have promised myself I will not allow that gradual decline - I love cooking, eat everything (in moderation), drink every day and don’t diet. I play tennis against a 46-year-old bloke every week.

My life is about saying ‘yes please’ to things, from choosing to work on different projects consistently and accepting challenges I’ve never done before, to regularly going out and spending time talking to people who are half my age.

I say ‘yes please’ far more than I ever say ‘no thanks’ (a very good little test to do at the end of a day).

All the above is good for mental stimulation. Nevertheless, whenever I read about ageing, it’s always in the context of bad news - inadequate pensions, the threat of Alzheimer’s and loneliness. I want my remaining years to be happy and glorious, but that’s going to need careful planning.

Lynda Gratton is a professor at the London Business School, an expert on the world of work who has produced bestsellers on the subject. She has co-written a book provocatively entitled The 100-Year Life, which looks at what we need to put in place to ensure our final years are, as she puts it, ‘a gift, not a curse’

For starters, how much longer can I expect to live? Both my parents died in their late 70s, but that’s not on my agenda. Advances in medicine, improved diet and healthcare mean there are more centenarians than ever, and half of today’s 20-year-olds have every chance of living to 100.

I could have another three decades to enjoy myself, but what quality of life can I expect in my late 90s?

In spite of hiking, tennis and freezing swims in the Channel, plus massive consumption of high-fibre grains, will my body turn into an engine which gradually runs out of steam?

In spite of hiking, tennis and freezing swims in the Channel, plus massive consumption of high-fibre grains, will my body turn into an engine which gradually runs out of steam? And, more importantly, will my mind start to unravel?

I can’t begin to contemplate such an existence. I plan to do everything in my power to ensure that I have as much fun as possible during my remaining years, as my life enters its ‘third act’.

Lynda Gratton is a professor at the London Business School, an expert on the world of work who has produced bestsellers on the subject. She has co-written a book provocatively entitled The 100-Year Life, which looks at what we need to put in place to ensure our final years are, as she puts it, ‘a gift, not a curse’.

Some people, like my late mum, view their old age fearfully and with dread. Lynda reckons that if we are prepared to be flexible, to learn new skills and not shut our minds down, we can enjoy longevity and create a time that is exciting and stimulating.

To live to 100 we must plan ahead and avoid falling into the same traps as our parents, who continually reminisced about the past and saw the same shrinking circle of friends, becoming increasingly anxious about trying anything new.

To be one of the growing army of new centenarians, you need to safeguard yourself financially but, equally important, you need to flex your mind and treat it like a set of muscles that constantly need to be exercised and toned, not allowed to atrophy.

Lynda reckons that if we are prepared to be flexible, to learn new skills and not shut our minds down, we can enjoy longevity and create a time that is exciting and stimulating

The 100-Year Life is full of advice but it’s an academic work, not exactly a light read. However, if you visit the website 100yearlife.com, which Lynda and her co-author, economist Andrew Scott, have set up, there is a simple diagnostic test which really makes you consider whether you are doing the right things to age successfully and productively, not loafing around watching Flog It in your trackie bottoms.

This is music to my ears.

I sit down with Lynda and attempt to respond to a range of deceptively simple statements, ticking a range of optional replies from ‘strongly disagree’ right through to ‘strongly agree’.

Of course, being more like a bolshie, ageing stroppy teenager, I start moaning about the questions and tick a few ‘neither agree or disagree’ boxes, but in general the exercise is extremely revealing.

It will make you stop and contemplate some uncomfortable truths. According to Lynda, there are three ways to live out the rest of your life, building on what she describes as your assets (mental and material), maintaining them or depleting them.

For the best chance of a happy and rewarding old age, I must ‘design the rest of my life’, as Lynda puts it.

One statement says ‘I’m keen to learn more about myself’. Personally, I know far too much about myself - as a writer and broadcaster, I could be accused of being an egomaniac whose special area of interest is ‘myself’.

Good ageing is not just about continuing to work, it’s making sure our leisure is mind-expanding

Another question asks whether ‘I am looking forward to slowing down and working less’ — an alien concept as far as I’m concerned. I work about the same number of hours a week as I did ten years ago, sometimes more.

Good ageing is not just about continuing to work, it’s making sure our leisure is mind-expanding. Equally, if you choose to work hard like me, you must factor in proper down-time, not time when you stare at a phone or answer emails.

There has to be sacrosanct time to enjoy Nature and feed your brain with nurturing experiences.

There are other questions which relate to stress, which I am particularly interested in as I’m planning to move house in the next month or so, and it’s said that moving and divorcing are the two most stressful things you can do in life.

Divorces haven’t really aged me. They have been liberating experiences, and I suspect a lot of older women would agree. But moving can be dreadful.

As a perfectionist and a bit of a hoarder, I can see that too much planning over transferring all my stuff is doomed to failure and will result in anger and frustration (very ageing!). But I have made an important decision - not to buy a flat in a block with people I don’t know, but a house with a garden in a quiet square, with real neighbours to whom I can talk as I age. Fingers crossed, the move will be a manageable process.

Getting set in your ways is not good if you want to live to 100 and Lynda tells me the new buzzword is ‘juvenescence’, the ability to think and act like a teenager, constantly re-inventing yourself, questioning rules and asking if there are new ways of doing things

Getting set in your ways is not good if you want to live to 100 and Lynda tells me the new buzzword is ‘juvenescence’, the ability to think and act like a teenager, constantly re-inventing yourself, questioning rules and asking if there are new ways of doing things.

I am this juvenescent person personified. Show me a rule or a queue and I’m asking ‘why?'.

But change for its own sake can be exhausting and unnecessary, like the time when I changed the sofa four times because I thought it didn’t look right and wasn’t the right colour. My partner nearly put a cushion over my head and stopped me getting to even 70.

The 100-Year Life: Living And Working In An Age of Longevity

Lynda says that to age best you need to retrain yourself for the new life you’ll be having, to be open to learning new skills.

I go a bit quiet at this point. This is where my new future career as a centenarian nearly hits the buffers.

I am pushed to think of a new skill I have mastered in the past two years, but I’ve been too busy. I announced that I would learn to drive a car with gears when I was 65 and I still haven’t got round to it. I still can only swim breaststroke, and I can’t dive - I’m too frightened my teeth could snap off when I hit the water.

Finally, after several minutes of contemplation (my new mindfulness skill), I realise that I have accomplished two new things in my late 60s. Number one, I can just about manage to live with a dog. My partner bought Badger the border terrier without consulting me. I can coexist with Badger for about five days before the dog hair and muddy paws start me nagging.

My other new skill is being nicer - not too much, but I’ve worked on being pleasant (although not to people who try to take my picture when I look like a tramp in Sainsbury’s).

If you’re not physically capable of a huge amount of exercise, brush up on your computing skills because the internet is your gateway to the wider world, knowledge and information. It gives you limitless horizons and you can Skype people anywhere.

Far too many older women can’t use a computer, which is shocking. Mind you, the fastest-growing group using online dating sites are the elderly, so clearly some pensioners are expert silver surfers.

Loneliness contributes to an early death as much as smoking and many medical conditions. It is the silent killer.

Half of those aged over 75 live alone and two fifths have only a television set for company. Starting a conversation requires a big effort but it is essential. And don’t fall out with older friends (like my mum did) over petty squabbles or imagined slights.

Lynda says that friends you’ve had for more than 50 years are worth a fortune in the bank of health. They are the only people who will tell you the truth.

I’ve known a group of pals since my early 20s, and we regularly meet for a meal and go on holiday together.

It is also important to have conversations with people right out of your age group - not babies but young adults, people who might encourage you to try something new or to see things differently.

Taking care to have variety in your daily experiences is essential if you want to reach 100.

Having completed the test, I am pleased to discover that my scores are high - but one unpleasant trait could stop me achieving my goal.

Moaning is a horrible by-product of ageing. It creeps up on you, and before long your face is set like a wrinkly gargoyle as you witter on about your arthritis, the weather, the news, politicians, the price of petrol, etc, etc. The list is endless.

I MUST NOT MOAN is going on my screensaver now, along with a chart of goals to achieve before I’m 100. See you at the finish line.

The 100-Year Life: Living And Working In An Age of Longevity, by Lynda Gratton and Andrew Scott, is published by Bloomsbury, £18.99.